For many years, microwave energy has been used for domestic cooking and processing of materials in industry. The advantages of heating with microwave energy have been well documented. One such advantage has been that less insulation is generally required around the heating cavity because the rise in temperature in a microwave cavity is substantially caused only by air cooling the magnetron and radiation from the heating body. For example, typical temperatures in a domestic microwave cooking oven range from 100.degree. to 150.degree. F. Even in industrial microwave applications, interior cavity temperatures, which are a function of the application, are frequently below 150.degree. F. With conventional domestic ovens utilizing gas or electric power, cooking temperatures commonly range from 200.degree. to 500.degree. F. with higher temperatures for broiling.
For some industrial microwave processing applications, it has been found to be desirable to provide a hot air blanket around the surfaces of the processing material. For example, in a conveyorized microwave system for vulcanizing rubber, a surface temperature in the range from approximately 170.degree. to 500.degree. F. is important to compensate for the surface heat lost by radiation. The hot air blanket helps provide for uniform curing through the rubber profile.
Accordingly, the apparatus should provide both microwave heating and hot air in the processing cavity. Including both of these heating means in a single cavity without violating any of the basic disciplines associated with each technology has resulted in excessive apparatus cost. Present methods commonly utilize an inner conductive enclosure to contain the microwave energy surrounded by layers of a standard glass insulation and a protective outer casing.